The quantity of residue generated by the incineration of municipal waste is far from negligible. In France alone, about six million tons of municipal waste are incinerated, which results in the production of nearly two million tons of ash, which must be disposed of in ways that are not harmful to the environment.
Studies have shown that coal fly ash, which includes many very soluble metallic salts, is susceptible of treatment. However, the treatment for coal fly ash is quite different from the treatment of ash from incineration of municipal waste, as the compositions of the two products are quite different.
Ash from municipal waste is not to be confused with coal fly ash, which is a well-known waste material usually recovered from the stack gases of pulverized coal-burning furnaces. Coal fly ash is a finely-divided material including some unburned carbon, but mostly comprised of various silica, alumina and iron compounds present in a variety of forms, including finely divided spherical glassy materials.
Disposal of ash from municipal waste is complicated by several types of reactions that occur with this ash. There is a reduction of organic compounds, which provokes release of hydrogen sulfide. Additionally, there is a risk of solubilizing dioxins. The most toxic of the dioxins, 2,3,7,8 TCDD, which is found in trace amounts in residues of ash from incineration of municipal waste, is slightly soluble in water, but may be mobilized by nonpolar organic compounds, such as oils. There is, of course, always a risk of solubilizing heavy metal, which, although precipitated in the form of the hydroxides in the initial basic ash, can be redissolved when the pH of the ash is reduced, for example by leaching.
Municipal solid waste varies in composition by city, by season of the year, and by weather conditions. Many of the components of municipal solid waste can be burned to produce fuel. However, other than clinker and smoke purification of waste products, an incinerator produces a large quantity of ash in the form of fine particles that are generally trapped using electrostatic devices. Of course, the composition of this ash varies depending upon the types of waste materials, the equipment used, and the operation conditions. The components of municipal waste generally comprise household waste, including food waste, yard waste, glass, metal, paper, plastics and textiles, wood, leather, rubber, and other waste materials, and the ash formed by incineration of these materials contains residues from them.
For example, in the case of incineration of household garbage, one ash had the following approximate composition: 23% Si, 7% Al, 4% Fe, 1% Pb, 1.9% Zn, 8% Ca, 2.5% Mg, 0.3% Ba, 4% K, 3% Na, 0.7% Ti, 0.03% Cd, 0.3% Sn, 0.1% Cr, 1% C1, 3% S, 3% Na, 3% N, 4% P, 5% organic carbon, and approximately 1% unburned materials.
Municipal waste ash that is stored in moist environments can generate considerable pollution due to leaching. In this way, for example, a representative sample having a pH of approximately 6.8 subjected to DIN standard 38414 salts out the following toxic elements by leaching, as expressed in parts per million: 6.3 ppm Pb, 0.2 ppm Cu, 60 ppm Cd, 900 ppm Zn, 0.2 ppm Fe.
Typical chemical compositions of coal fly ash are shown below:
______________________________________ Fly Ash Class Compounds Class F Class C ______________________________________ SiO.sub.2 54.9 39.9 Al.sub.2 O.sub.3 25.8 16.7 Fe.sub.2 O.sub.3 6.9 5.8 CaO 8.7 24.3 MgO 1.8 14.6 SO.sub.3 0.6 3.3 ______________________________________
Some processes have been proposed to insolubilize municipal waste ash from incineration, for example, by adding to it significant quantities of lime, cement or mixtures of cement and sodium silicate. The compounds produced by the use of such treatments are highly basic (pH generally greater than 12) and tend to solubilize amphoteric elements, in particular lead and zinc. Another problem may arise in the untimely production of hydrogen according to the reaction: EQU OH.sup.- +H.sub.2 O+Al.fwdarw.AlO.sub.2.sup.- +3/2 H.sub.2